1. Field of the Invention
The present invention relates to a wheel loader.
2. Related Art
In a wheel loader which is provided with a so-called HST (hydro-static transmission) circuit, a hydraulic pump is driven by an engine, and hydraulic oil which is discharged from the hydraulic pump is supplied to a hydraulic motor. Then, a vehicle moves due to moving wheels being driven by the hydraulic motor.
In the past, as a wheel loader as described above, a wheel loader is known which electronically controls a capacity of the hydraulic motor as shown in Japanese Unexamined Patent Application Publication No. 2004-144254. The wheel loader is provided with a hydraulic motor, a cylinder, and a control valve. The cylinder has a cylinder body and a piston rod which extends and retreats with regard to the cylinder body, and an angle of an inclined shaft, that is, an inclining angle of the hydraulic motor is changed by movement of the piston rod. In addition, the piston rod is connected with the control valve. The control valve is an electromagnetic control valve which is electrically controlled by a control section. Accordingly, in the wheel loader, it is possible to arbitrarily change the capacity of the hydraulic motor by controlling the cylinder by electrically controlling the control valve.
As described above, the control section outputs a predetermined command signal to the control valve in a case where the capacity of the hydraulic motor is electronically controlled. The command signal is set using feedback control. In detail, actual hydraulic drive pressure which drives the hydraulic motor is detected and the command signal is set such that the actual hydraulic drive pressure is closer to a predetermined target hydraulic drive. Here, resistance of the hydraulic oil in a hydraulic circuit is large since viscosity of the hydraulic oil is large in a case where a temperature of the hydraulic oil is reduced. As a result, a response delay occurs in the hydraulic motor with regard to changes in the command signal to the control valve. As a result, there is a concern in that a phenomenon is repeated such that the actual hydraulic drive pressure is larger than or smaller than the target hydraulic drive pressure and hunting oscillation occurs along with this due to repetition of unstable increasing and decreasing of the rotations of the engine.
An object of the present invention is to suppress occurrence of hunting oscillation.
(1) A wheel loader according to an aspect of the present invention is provided with an engine, a hydraulic pump, a hydraulic motor, a motor capacity control section, moving wheels, an oil temperature detecting section, an accelerator opening detecting section, a forward and reverse switching operation member, an operation detecting section, and a control section. The hydraulic pump is driven by the engine. The hydraulic motor is a variable capacity hydraulic motor which is driven by hydraulic oil which is discharged from the hydraulic pump. The motor capacity control section controls the capacity of the hydraulic motor. The moving wheels are driven by the hydraulic motor. The oil temperature detecting section detects the temperature of the hydraulic oil. The accelerator opening detecting section detects an accelerator opening degree. The forward and reverse switching operation member performs a switching operation in order to output an instruction for switching a moving state of the vehicle to any one of a forward state, a reverse state or a neutral state. The operation detecting section detects a switching instruction from the forward and reverse switching operation member. The control section selectively executes a first control and a second control in a case where the accelerator opening degree is less than a first opening which is set in advance and a switching operation is performed by the forward and reverse switching operation member. The control section controls the motor capacity control section so as to maintain the capacity of the hydraulic motor before and after the switching operation when the first control is executed. The control section controls the motor capacity control section such that the capacity of the hydraulic motor is a minimum value which is set in advance at each speed level when the second control is executed. In addition, the control section executes the first control in a case where the temperature of the hydraulic oil is equal to or less than a first temperature which is set in advance. The control section executes the second control in a case where the temperature of the hydraulic oil is higher than the first temperature.
According to such a configuration, first, it is possible to prevent rapid deceleration of the wheel loader since the control section executes the first control or the second control when the switching operation is performed and the accelerator opening degree is less than the first opening. In detail, it is possible to prevent rapid deceleration of the wheel loader using the switching operation since the capacity of the hydraulic motor is maintained before and after the switching operation when the control section executes the first control. In addition, it is possible to prevent rapid deceleration of the wheel loader using the switching operation since the capacity of the hydraulic motor is the minimum value when the control section executes the second control.
Here, the control section executes the first control in a case where the temperature of the hydraulic oil is equal to or less than the first temperature which is set in advance, that is, a case such that the temperature of the hydraulic oil is a low temperature and it is easy for a response delay to occur in the hydraulic motor. The capacity of the hydraulic motor is maintained before and after the switching operation using the forward and reverse switching operation member when the control section executes the first control. As a result, it is possible for the capacity of the hydraulic motor after the first control is released to not rapidly become large compared to the capacity of the hydraulic motor when the first control is executed. Therefore it is possible to suppress the occurrence of hunting oscillation. In addition, it is difficult for hunting oscillation to occur even when the control section executes the second control since it is difficult for a response delay to occur in the hydraulic motor in a case where the temperature of the hydraulic oil is higher than the first temperature which is set in advance.
(2) Preferably, it is possible for the control section to execute maximum speed variable control where a maximum speed of a vehicle is changed on a plurality of levels due to a change in a minimum value of the capacity of the hydraulic motor. According to this configuration, it is possible for the operator to easily move at a desired speed.
(3) Preferably, the control section performs the first control in a case of selecting the maximum speed level where the maximum speed is the highest in maximum speed variable control. When the maximum speed level is selected, it is easy for hunting oscillation to occur since the minimum value of the capacity of the hydraulic motor is minimized. It is possible to efficiently suppress hunting oscillation by the control section executing the first control in the case where it is easy for hunting oscillation to occur. Here, it is preferable that the control section executes the second control since it is difficult for hunting oscillation to occur in a case where the temperature of the hydraulic oil is higher than the first temperature even in a case where the maximum speed level is selected.
(4) Preferably, the wheel loader is further provided with a vehicle speed detecting section which detects vehicle speed. According to this configuration, it is possible for the control section to execute various controls according to vehicle speed.
(5) Preferably, the control section releases execution of the first control or the second control when the vehicle speed reaches 0 km/h. According to this configuration, it is possible for the control section to release execution of the first control or the second control at a more appropriate timing.
(6) Preferably, the wheel loader is further provided with a pressure detecting section which detects hydraulic drive pressure which is pressure of the hydraulic oil which drives the hydraulic motor. Then, the control section executes a third control for controlling the motor capacity control section such that the hydraulic drive pressure is closer to a predetermined target hydraulic drive pressure in a case where the switching operation is not performed by the forward and reverse switching operation member. According to this configuration, the control section executes the third control since the wheel loader does not rapidly decelerate in a case where the switching operation is performed.
(7) Preferably, the control section controls the motor capacity control section in the third control such that the capacity of the hydraulic motor is equal to or more than a lower limit value which is set in advance based on the vehicle speed. It is easy for hunting oscillation to occur when the capacity of the hydraulic motor is small when the vehicle speed is low. In contrast to this, it is possible to suppress the occurrence of hunting oscillation by the control section controlling the motor capacity control section such that the capacity of the hydraulic motor is equal to or more than a lower limit value which is set in advance based on the vehicle speed.
(8) Preferably, the first temperature is 10 degrees. It is possible to more appropriately suppress hunting oscillation by setting the first temperature to 10 degrees since it is easy for a response delay to occur in the hydraulic motor when the temperature of the hydraulic oil is equal to or lower than 10 degrees.
(9) Preferably, the first opening is 50%. It is possible for the control section to determine that the operator does not desire rapid deceleration and to execute the first control or the second control described above when the accelerator opening degree is less than 50%.
According to the present invention, it is possible to suppress occurrence of hunting oscillation.